The train car shown in the final shot of "Put on Your Sunday Clothes" is from the Strasburg Rail Road, an active steam museum short line still in existence. The "Hello Dolly" car is open to the public.
Many of the taller building facades constructed on the Los Angeles front lot of 20th Century Fox as part of the outdoor sets (representing 1890 New York City) concealed the towers of then-new Century City--constructed on the former back lot of the studio..
During the number "Put on Your Sunday Clothes" - a few minutes into the number - Dolly, Emergarde and Ambrose do a high step down a flight of stairs in front of Vandengelder's Hay and Feed. The camera changes angle and across the bay, where there is a highway, there are modern era (1969) cars and trucks whizzing by.
During the 'Elegance' sequence, the hem on the back Irene's dress gets dirtier and dirtier as they walk to Harmonia Gardens. Inside the restaurant, the hemline is spotless.